Drilling fluids are reputed as the blood for bore wells. Good drilling fluid techniques are one of the important guarantees for safe, high-quality, efficient, and quick well drilling production. Drilling fluids are required in the development of oil fields.
Effective reservoir protection is an effective guarantee for improving the rate of oil and gas recovery, so that the skin factor can be an optimal negative value. If the drilling fluid is designed or used inappropriately in the well drilling process, the liquid and solid in the drilling fluid may intrude into the oil reservoir and have physical and chemical reactions with clay and other minerals in the oil and gas layer. Consequently, the permeability in the oil layer in the immediate vicinity of the well may be decreased severely, and the resistance against air and gas flow towards the bottom of the well may be increased, resulting in reduced oil yield.
Usually, reservoirs that are different in permeability from each other have different structural characteristics, and different drilling fluid systems have to be applied for them. For example, the permeability of high/ultra-high permeability reservoirs is 500 mD or above. For high-permeability reservoirs, the permeability can be 500-2,000 mD; for ultra-high permeability reservoirs, the permeability can be 2,000 mD or above. The permeability of medium-permeability reservoirs is 100-200 mD. The permeability of low-permeability reservoirs is 10˜100 mD; the permeability of ultra-low permeability reservoirs is lower than 10 mD. Damages to the reservoir may occur owing to many factors, for example, blockage resulted from the solid particles carried by the drilling fluid and particles produced in the drilling process, reservoir damages resulted from hydrated expansion, dispersion and migration of clay minerals; the intrusion of the working filtrate (e.g., drilling fluid) into the reservoir may result in damages such as clay swelling, dispersion and migration, and water blocking, and have impacts on the accuracy of well logging; especially, it will be more difficult to drain off the drilling fluid if the foreign fluid in the drilling fluid has physical and chemical reactions with the reservoir fluid or reservoir rock; consequently, the oil and gas resources in the reservoir may be contaminated. For different reservoirs, damages may occur at different levels. Therefore, a protecting agent that is applicable to a specific reservoir structure often is inapplicable to other reservoir structures.
Up to now, though long-time researches on reservoir protective drilling fluid techniques have been made in China and foreign countries, and techniques such as shielded temporary plugging technique, fractal geometry-based temporary plugging technique, D90 ideal temporary plugging technique, broad-spectrum temporary plugging technique, alkali soluble micron-sized cellulose temporary plugging technique, D50 temporary plugging technique, filming technique, oil film technique, etc., have been developed, the protective effect of these techniques is not ideal for protection of reservoirs different in permeability. These techniques have to be improved further. The root cause is that these techniques don't take consideration of the reservoir damages such as water blocking and water sensitivity incurred by spurt loss, and the plugging effect of these techniques should be further improved under some conditions.
The existing drilling fluids have properties that can essentially meet the requirement for cuttings carrying and well wall stability, but don't provide an ideal reservoir protection effect, and the core permeability recovery value of the reservoir is low. Though temporary plugging agents are added in these drilling fluids, the plugging performance is not high because the matching relation between the pore throat size and the particle size of temporary plugging agent is not taken into consideration fully. Consequently, the filtrate may intrude into the reservoir and thereby results in damages such as water sensitivity and water blocking, etc.